The present invention relates to magnetic-induction devices. More specifically, the invention is directed to a reduced-cost core for an electrical-power transformer.
Electrical-power transformers are used extensively in electrical and electronic applications. Transformers transfer electric energy from one circuit to another circuit through magnetic induction. Transformers are utilized to step electrical voltages up or down, to couple signal energy from one stage to another, and to match the impedances of interconnected electrical or electronic components. Transformers are also used to sense current, and to power electronic trip units for circuit interrupters. Transformers may also be employed in solenoid-equipped magnetic circuits, and in electric motors.
A typical transformer includes two or more multi-turned coils of wire commonly referred to as xe2x80x9cphase windings.xe2x80x9d The phase windings are placed in close proximity so that the magnetic fields generated by each winding are coupled when the transformer is energized. Most transformers have a primary winding and a secondary winding. The output voltage of a transformer can be increased or decreased by varying the number of turns in the primary winding in relation to the number of turns in the secondary winding.
The magnetic field generated by the current passing through the primary winding is typically concentrated by winding the primary and secondary coils on a core of magnetic material. This arrangement increases the level of induction in the primary and secondary windings so that the windings can be formed from a smaller number of turns while still maintaining a given level of magnetic-flux. In addition, the use of a magnetic core having a continuous magnetic path ensures that virtually all of the magnetic field established by the current in the primary winding is induced in the secondary winding.
An alternating current flows through the primary winding when an alternating voltage is applied to the winding. The value of this current is limited by the level of induction in the winding. The current produces an alternating magnetomotive force that, in turn, creates an alternating magnetic flux. The magnetic flux is constrained within the core of the transformer and induces a voltage across in the secondary winding. This voltage produces an alternating current when the secondary winding is connected to an electrical load. The load current in the secondary winding produces its own magnetomotive force that, in turn, creates a further alternating flux that is magnetically coupled to the primary winding. A load current then flows in the primary winding. This current is of sufficient magnitude to balance the magnetomotive force produced by the secondary load current. Thus, the primary winding carries both magnetizing and load currents, the secondary winding carries a load current, and the core carries only the flux produced by the magnetizing current.
Modern transformers generally operate with a high degree of efficiency. All magnetic devices such as transformers, however, undergo losses because some fraction of the input energy to the device is inevitably converted into unwanted heat. The most obvious type of unwanted heat generation is ohmic heating that occurs in the phase windings due to the resistance of the windings.
Two other forms of losses occur in the transformer core as a result of hysteresis and eddy currents. These losses are collectively referred to as xe2x80x9ccore losses.xe2x80x9d Hysteresis losses represent the energy required to overcome molecular friction within the core. This friction is caused by the many reversals that the molecules in the core undergo every second due to the effects of the alternating magnetic flux. Hysteresis losses are typically reduced by constructing the core from special materials such as textured silicon steel. Eddy current losses are ohmic losses that result from the circulation of eddy currents within the core. The eddy currents are produced as the core is cut by the magnetic flux generated in the windings. Eddy-current losses are typically reduced by forming the core from thin laminae of iron or steel.
Transformer cores commonly comprise two or more magnetic loops arranged side by side. For example, FIG. 1 depicts a conventional three-phase transformer 98 comprising a core 100 having four magnetic loops 102. The loops 102 are arranged side by side so as to form three winding legs 110. A phase winding 112 is disposed around each winding leg 110 so that each phase winding 112 is inductively coupled to its respective winding leg 110 when the transformer 98 is energized.
Each of the magnetic loops 102 is wound from a narrow, thin strip of magnetic material such as textured silicon steel or an amorphous alloy. In other words, each of the magnetic loops 102 is made up of a plurality of laminae 103 formed by a single winding of magnetic material. FIGS. 1A and 1B are diagrammatic illustrations depicting portions of the laminae 103 of two adjacent loops 102. The sizes of the laminae 103 are exaggerated in these figures, for clarity.
The cores losses that occur in each loop 102, in general, are proportionate to the thickness of the strip material from which the loop 102 is formed (in particular, the thinner material provides a smaller flow-path for the loss-inducing eddy currents). The cost of the thinner material, however, is generally higher than that of the thicker material. Hence, an optimal transformer design must balance material costs against the need to minimize core losses. Manufacturers of electrical-power transformers are under constant pressure from their customers to minimize both the purchase cost and the operating costs of their products. Hence, an ongoing need exists for reduced-cost, efficient transformers.
Each of the four magnetic loops 102 is typically formed from strips of material having a substantially identical thickness (see, e.g., FIGS. 1A and 1B). This practice is followed in order to equalize the level of induction and the resulting core losses in each of the winding legs 110. The noted practice is dictated by a widely-held belief among skilled transformer designers that the overall core losses in a core such as the core 100 are equal to the numerical average of the core losses in the individual magnetic loops 102, regardless of whether the loops 102 are operating under identical conditions. Hence, a potential cost savings associated with reducing the thickness of the materials from which one or more, but not all, of the loops 102 are formed cannot be realized according to the currently-accepted teachings in the art of transformer design.
An object of the present invention is to provide an electrical-power transformer having a reduced-cost core. In accordance with this object, a presently-preferred embodiment of the invention provides an electrical-power transformer comprising a core comprising a first magnetic loop including a first winding formed by a first strip of magnetic material having a first thickness, and a second magnetic loop including a second winding formed by a second strip of magnetic material having a second thickness. The second thickness is less than the first thickness, and the first and the second magnetic loops are positioned substantially side by side so that the first and the second magnetic loops form a winding leg. The transformer also comprises a phase winding that encircles the winding leg so that the phase winding and the winding leg are inductively coupled when the transformer is energized.
Further in accordance with the above-noted object, another presently-preferred embodiment of the invention provides an electrical-power transformer comprising a core comprising a first magnetic loop including a first plurality of laminae each having a first thickness, and a second magnetic loop including a second plurality of laminae each having a second thickness. The second thickness is less than the first thickness, and the first and the second magnetic loops are positioned substantially side by side so that the first and the second magnetic loops form a winding leg. The transformer also comprises a phase winding that encircles the winding leg so that the phase winding and the winding leg are inductively coupled when the transformer is energized.
Another object of the present invention is to provide a reduced-cost core for an electrical-power transformer. In accordance with this object, another presently-preferred embodiment of the invention provides a core for a three-phase electrical-power transformer, comprising a first, a second, a third, and a fourth magnetic loop. The first, second, third, and fourth magnetic loops are positioned substantially side by side in the recited order to provide three winding legs and two outer legs. The first magnetic loop includes a first winding formed by a first strip of magnetic material having a first thickness, and the second magnetic loop includes a second winding formed by a second strip of magnetic material having a second thickness. The third magnetic loop includes a third winding formed by a third strip of magnetic material having substantially the second thickness, and the fourth magnetic loop includes a fourth winding formed by a fourth strip of magnetic material having substantially the first thickness.
Further in accordance with the above-noted object of providing a reduced-cost transformer core, another presently-preferred embodiment of the invention provides a core for a three-phase electrical-power transformer, comprising a first, a second, a third, and a fourth magnetic loop. The first, second, third, and fourth magnetic loops are positioned substantially side by side in the recited order to provide three winding legs and two outer legs. The first magnetic loop includes a first plurality of laminae each having a first thickness, and the second magnetic loop includes a second plurality of laminae each having a second thickness. The third magnetic loop includes a third plurality of laminae each having substantially the second thickness, and the fourth magnetic loop includes a fourth plurality of laminae each having substantially the first thickness.
A further object of the present invention is to provide a reduced-cost core for a magnetic-induction device. In accordance with this object, another presently-preferred embodiment of the invention provides a core for a magnetic-induction device comprising a first magnetic loop including a first winding formed by a first strip of magnetic material having a first thickness, and a second magnetic loop including a second winding formed by a second strip of magnetic material having a second thickness. The second thickness is less than the first thickness, and the first and the second magnetic loops are positioned substantially side by side so that the first and the second magnetic loops form a winding leg for a phase winding.
Further in accordance with the above-noted object of providing a reduced-cost core for a magnetic-induction device, another presently-preferred embodiment of the invention provides a core for a magnetic-induction device comprising a first magnetic loop including a first plurality of laminae each having a first thickness, and a second magnetic loop including a second plurality of laminae each having a second thickness. The second thickness is less than the first thickness, and the first and the second magnetic loops are positioned substantially side by side so that the first and the second magnetic loops form a winding leg for a phase winding.